Ekant Veer at the University of Bath said they were more effective than campaigns telling overweight teens they needed to slim down.

Over 300 obese or overweight school children aged 13 to 18 were studied.

Mr Veer said shows such as Mr Oliver's were an "starting point" for encouraging children to be healthier.

In the study the teenagers were split into two groups, one of which was asked to draw a picture of themselves - subtly leading them to think about their body image.

Motivation

Both groups were then shown advertising posters on having a healthy breakfast - one urging children to slim down, the other providing educational information about the benefits of eating well.

Three-quarters of the children who had been asked to draw pictures of themselves said they felt encouraged to eat more healthily and do more exercise after seeing the adverts.

Getting young people to think about themselves frequently makes them much more receptive to campaigns giving information about how to eat more healthily

Ekant Veer

But of those who had not been asked to draw a picture, only 58% said the adverts had an effect.

Mr Veers, a marketing lecturer at the university, said the key message of the findings was that campaigns which portray a negative image of overweight people are not actually encouraging them to slim down.

"We're talking about healthy living but we're not talking about fat or thin," Mr Veers told the BBC.

"This research shows that getting young people to think about themselves frequently makes them much more receptive to campaigns giving information about how to eat more healthily and to exercise," he said.

"TV shows such as those involving Jamie Oliver and school dinners are an excellent starting point since they will make school children think about their weight, without making children feel like they are not attractive or are worthless," he added.

"These types of messages don't work."

Dr David Haslam, clinical director of the National Obesity Forum said it would be interesting to see whether the findings would work in practice.

He said focusing on body image was acceptable "as long as it's done in a neutral and positive environment".

"To have an image of your body is perfectly normal. It's when the image is abnormal that it goes wrong," he said.

"If you lay the foundations down right.. then you're protecting against a both anorexia and obesity."

A follow-up study is underway to determine whether the campaigns are effective long-term in helping the teenagers lose weight.